Re: EQN build thread
« Reply #620 on: May 06, 2019, 06:03:31 PM »
Thanks for your response Paul! I have read through all the posts on this thread a few times and have also successfully built 4 EZ1290 preamps so I do know what to look for when it comes to achieving proper bias.
The problem I'm having now is that when I do achieve proper bias with RV5, I get low frequency noise that only goes away when i turn RV5 CW, but when I do so, I get very uneven clipping on my output waveform (only clipping on the negative swings).
I've double checked that I stuffed the PCBs (opamps and main boards) correctly and I'm using MPSA18 transistors in lieu of BC184c (flipped around 180 degrees of course).

If the wiper for RV5 is too far to one side (CW?) you will get oscillation. Put RV5 in the middle of it's range and do the same for RV4. Read some of the posts on this thread about how the input trim affects the bias and then proceed accordingly.



Potato Cakes

Re: EQN build thread
« Reply #621 on: May 06, 2019, 09:52:04 PM »
When you adjust RV5 to get rid of noise, have you then tried adjusting RV4 to change the input gain and see if that helps? I distinctly remember that being the issue for someone building these. I recently finished my pair and I had the same issue with the LF oscillation because RV5 was turned too far CCW. One is built with MPSA18 and the other with BC184C's, both running on +24V (51X).

See if RV4 is the answer and let us know.



Re: EQN build thread
« Reply #622 on: May 19, 2019, 05:57:23 PM »
Hi again Paul,

Still having some issues with the EQNs... I had some BC184c's in the mail on the way to me already so I put them in one of the EQNs I have.  I also reflowed some joints around the output stage. Whereas before I was getting loud oscillation at the proper bias current, I'm not getting softer noise (around -63dbFS into my interface). It's definitely better now, but I have verified that the issue is coming from the output stage by listening to the positive leg of C48 - here I have no low frequency noise like I do on the output.

To answer your previous question, it doesn't seem that RV4 (the volume trimmer) affects the noise at all.

Since I've now replaced the two BC184cs in the output section, are there any other components around there that may be bad like a noisey resistor or capacitor?

Thank you,

Re: EQN build thread
« Reply #623 on: May 27, 2019, 11:24:21 PM »
I've done more work on the boards and discovered that the issue is certainly in the DC-DC circuit. When bypassing the DC-DC converter and powering the boards with 24v, I get clean operation!

I'm assuming it will be an issue with the TDK/Lamda converter so hopefully I'll have replacements for those soon to verify, otherwise I'm looking towards Q1 and surrounding components...

thank you


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